Responsive all the things (Admin UI responsive, mange content on any screen size)

Flexible Content Delivery (RESTful API, content as a service, pull content into the system, now you can have content back-end with a flexible front-end)

Views in core

A bunch of pretty much always installed contrib modules in core

Comprehensive content modeling out of the box with entities, fields, and views

Customization of content pages and even forms and administrative pages via the administrative interface

Full translatability and localization out of the box

Reliable configuration management for safe and straightforward deployment of changes between environments

Mobile-first, responsive, HTML5 output

REST-first native web services

Enhanced accessibility and WAI-ARIA compliance

Modern PHP standards and practices, with integration of popular libraries such as Composer, Symfony2, Guzzle, and Twig

Significantly improved front-end performance out of the box

Enhanced caching and best-of-class integration with CDNs and reverse proxies

Full compatibility with PHP7, and the PostgreSQL and SQLite databases

What about Drupal 6? Well it is at its end of life now D8 has been released. But thats okay you still have until February 24th 2016.
Drupal 6 will no longer be supported by the community at large. The community at large will no longer be creating new projects, fixing bugs in existing projects, writing documentation, etc. around Drupal 6.
There will be no more core commits on Drupal 6.x to the official tree. (see What if I have a Drupal 6 site still)
The security team will no longer provide support or Security Advisories for Drupal 6
All Drupal 6 releases on project pages will be flagged as not supported.
At some point in the future update status may stop working for Drupal 6 sites.

Foundation 6
foundation.zurb.com

motion ui (Easily Create Animations and Transitions, This Sass library includes more than two dozen built-in transition and animation classes.)

50% Code Reduction - Half the Size of Foundation 5. Total filesize of every component and class now weighs in at 60KB CSS & 84KB JS, with plenty of room to make them even smaller when selectively removing unused components.

A11y Friendly - The Base for Fully Accessible Sites. All code snippets come with ARIA attributes and roles along with instructions on how to properly use these components. This helps ensure that every website built on Foundation 6 can be used anywhere, on any device, by anyone.

Fewer Style Overrides - The Styles You Need. None That You Don't. The base styles act as a coded wireframe rather than a final design. Simpler CSS styles allow you to more easily modify them to fit your brand.

Customizable Sass Grid - Any Combination of Columns You Need. The Sass grid mixins have been made smartly to give you even more flexibility to customize the grids with any number of columns. (more like susy)

breakpoint mixin (similar how bourbon and neat work)

RTL Feature. Foundation supports right-to-left languages to help designers around the world design great products faster

Rails friendly. You can use Foundation with any back end, but it’s especially well-tuned for Rails implementations.

ZURB Development Stack - Prototype With the Tools ZURB Uses . Use the same template that ZURB uses on all our client projects. This starter template is a souped up stack complete with a custom static site generator to help flatten files into single HTML documents. It gives you 'Handlebars' templating, UglifyJS, UnCSS, and image compression and Sass as well.

Navigation has been split up to be far more flexible than previous, this was a big and impressive change

lots of new sass features used, so you will need a fairly new version of sass